Proposals have been made to detect the approach of a metal first part relative to a fixed second part, by observing the distortion of a magnetic field associated with the second part as the metal part approaches.
Advantageously, such detection may be performed by means of a Hall effect probe.
On this subject, reference may usefully be made to document ETZ-A, vol. 83, no. 11, May 21, 1962 pages 367-372. (Ch.J. Lippmann).
Such a method cannot immediately be applied to detecting the position of a vehicle axle with respect to a rail, since if the probe is connected to the axle, its height is not constant with respect to the rail due to the movements of the vehicle.
It is an object of the present invention to provide a sensor whose measurement signal is independent of the height at which the probe is located.
It is a further object of the invention to provide a sensor whose measurement signal is unaffected by the return current which flows in the railway track.